A burst of neutrinos – difficult to observe particles that can traverse the entire Universe – has just been tracked back to the flaring of a black hole jet at the centre of a galaxy four billion lightyears away.
This is only the second time that neutrinos have been detected outside of our Solar System and the first time they have
been traced back to the centre of another galaxy.
The discovery could help astronomers solve the century old conundrum of where cosmic rays come from.
Cosmic rays are particles with enough energy to travel vast distances across the Universe.
Unfortunately, their flight is deflected by magnetic fields they meet along the way, meaning the direction they arrive at Earth from doesn’t necessarily match with where they were created.
This means astronomers can’t directly trace their path back to their origin.
But cosmic rays are not born alone.
When they are created, uncharged sub-atomic particles called neutrinos form alongside.
As neutrinos are unaffected by magnetic fields, astronomers can look at their flight path to discover where they were created.
The neutrino burst was detected with the IceCube telescope in the Antarctic on 22 September 2017, and the results were published on 12 July 2018.
The team was able to use this observation to work out where on the sky the neutrinos came from.
The location was investigated with two gamma-ray telescopes: the Neils Gehrels Swift Observatory in space, and the High Energy Stereoscopic System on the ground.
These found that a blazar – a galaxy with a rapidly spinning black hole that fires out jets of particles– had flared in the region at the same time the neutrinos were emitted, suggesting that it was the source.
"Neutrinos rarely interact with matter," says Paul O’Brien, the head of Physics and Astronomy at the University of Leicester and a member of the observing team.
"To detect them at all from the cosmos is amazing, but to have a possible source identified is a triumph.
This result will allow us to study the most distant, powerful energy sources in the Universe in a completely new way."
"This event – the first time we've been able to associate light with the source of a high-energy neutrino – occurred less than 5 weeks after the first joint detection of light and gravitational waves," says Phil Evans, the development scientist for Swift.
"We truly are entering the era of multi-messenger astronomy."
